A Study of Historic Earthquakes of the Prince William Sound, Alaska, Region

Abstract

We have investigated the character of seismicity that occurred in the Prince William Sound/Cook Inlet region during ∼40 years prior to the 1964 Great Alaskan earthquake. We have relocated all events of magnitude > 5.5. Focal mechanisms have been determined from body-waveform modeling and first-motion analysis for the larger (generally magnitude > 6.3) events. The results suggest a number of similarities and differences between pre- and post-1964 mainshock seismicity. Similarities include the persistence of normal faulting at depths of 40 to 60 km within the subducted slab in both the Tazlina Glacier and Columbia Bay regions north of Prince William Sound and among deeper events within northern Cook Inlet. Differences include a higher level of shallow seismicity (<40 km depth) in the Upper Cook Inlet region and north of Cook Inlet prior to 1964, higher levels of seismicity within the subducting plate in the south-central and eastern Kenai Peninsula prior to 1964, and a lower level of seismicity in the offshore Prince William Sound region prior to 1964. Much of the lower plate seismicity for the past 70 years appears to cluster immediately downdip of the 1964 asperity. Two of the events in this study caused notable damage (Modified Mercalli intensity VII to VIII) in the Anchorage area. The first, in 1933 ( M w 6.9) appears to have occurred at ∼9 km depth along a strike-slip fault within northern Cook Inlet. The second, in 1954 ( M w 6.7) occurred within the subducting plate at ∼60 km depth beneath the northern Kenai Peninsula. Analysis of these events, as well as smaller events we have studied, will greatly improve models of seismic hazard for the Anchorage region. Manuscript received 18 September 2000.